statmount: retrieve security mount options

Add the ability to retrieve security mount options. Keep them separate
from filesystem specific mount options so it's easy to tell them apart.
Also allow to retrieve them separate from other mount options as most of
the time users won't be interested in security specific mount options.

Link: https://lore.kernel.org/r/20241114-radtour-ofenrohr-ff34b567b40a@brauner


Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>